There is another option which is being discussed in Norwegian media now - that the Russians have used the pipelines essentially for storage of excess gas (Seeing as the pipelines are basically, combined, a 5M cubic meter pressure vessel) That would go a long way towards explaining how several ruptures happened in relatively short succession - pipes being loaded up with too much gas (after all, the more gas stored, the more money in the bank for later on!) Once the first one burst, someone probably re-evaluated that idea, but letting off gas in sufficient quantities fast enough may not have been possible. Of course, if this was indeed the case, I assume it will very soon be very evident - I can't imagine that the rupture caused by an external force will be indistinguishable from that of it rupturing due to overpressure.